Frequency modulators with compensation for variations in modulation sensitivity

• US 4,309,674 A
• Filed: 08/31/1979
• Issued: 01/05/1982
• Est. Priority Date: 09/02/1978
• Status: Expired due to Term

• Current Assignee: Marconi Instruments Ltd. (Cobham PLC)
• Original Assignee: Marconi Instruments Ltd. (Cobham PLC)
• Inventors: Owen, David P.
• Primary Examiner(s): Grimm, Siegfried H.
• Application Number:
  US06/071,573
• Time in Patent Office: 858 Days
• Field of Search: 332/18, 332/19, 332/30 V, 455/110, 455/113, 455/42, 455/63
• US Class Current: 332/124
• CPC Class Codes: H03C 2200/0029 Memory circuits, e.g. ROMs, RAMs, EPROMs, latches, shift registers
• CPC Class Codes: H03C 2200/005 Modulation sensitivity
• CPC Class Codes: H03C 3/095 applying frequency modulation to the loop in front of the voltage controlled oscillator
• CPC Class Codes: H03C 3/0991 including calibration means or calibration methods

Abstract

In a frequency modulation system, the frequency modulation sensitivity of an oscillator is itself a function of the carrier frequency of the oscillator. The frequency modulation sensitivity is measured and stored for particular carrier frequencies, and subsequently used to control the amplitude of the modulation signal, so that when a frequency modulation signal is fed to the oscillator, a frequency deviation is obtained which is accurately related to the amplitude of the modulation signal.

Claims

1. A signal generator for producing a frequency modulated signal including a variable frequency oscillator whose frequency of oscillation is variable about a selected carrier frequency in accordance with a signal representative of a required frequency modulation, means for determining the frequency modulation sensitivity of the oscillator at the selected carrier frequency and means for controlling the amplitude of the representative signal in dependence thereon, the frequency modulation sensitivity of the oscillator being determined by measuring the extent to which changes in the frequency of oscillation are dependent on changes in the level of a frequency control signal fed to it, the oscillator being a free running oscillator and there being provided means for altering the amplitude of a frequency control signal by a predetermined step size to produce a resultant frequency change in the oscillator output, means for bringing the oscillator output frequency back to its initial value and means for deriving therefrom the frequency modulation sensitivity, the means for bringing the oscillator output frequency back to its initial value including an amplitude signal multiplier which is arranged to multiply the frequency control signal by a variable factor, the signal multiplier comprising a multiplying digital-to-analogue converter to which the variable factor is applied in digital form.

2. A signal generator as claimed in claim 1 and wherein the frequency modulation sensitivity is determined for a plurality of carrier frequencies, and the respective values thereof stored and subsequently used to control the amplitude of the signal representative of the required degree of modulation when a corresponding carrier frequency is selected.

3. A method of producing a frequency modulated signal by controlling the output of a variable frequency oscillator within an operating frequency range thereof, which comprises the steps of

(a) measuring the frequency modulation sensitivity of the oscillator over said operating frequency range;

(b) storing a signal indicative of said frequency modulation sensitivity over said operating frequency range;

(c) generating a frequency modulated signal by controlling the output of said variable frequency oscillator within said operating frequency range in accord with a modulating signal modified by said stored signal so as to compensate for non-linearity in the modulation sensitivity; and

(d) repeating steps (a) and (b) during times other than during step (c) whereby frequency modulation sensitivity of said oscillator is repetitively updated.

4. A method of producing a frequency modulated signal by controlling the output of a variable frequency oscillator over a selected one of a plurality of operating frequency ranges thereof, each range corresponding to a selected different carrier frequency which comprises the steps of

(a) measuring the frequency modulation sensitivity of the oscillator over each operating frequency range corresponding to each of said different carrier frequencies;

(b) storing signals indicative of each frequency modulation sensitivity measured in step (a);

(c) generating a frequency modulated signal by controlling the output of said variable frequency oscillator about a selected carrier frequency over the corresponding operating frequency range in accord with a modulating signal modified by that one of said stored signals corresponding to the selected carrier frequency so as to compensate for the nonlinearity of the modulation sensitivity; and

(d) repeating steps (a) and (b) during times other than during step (c) whereby frequency modulation sensitivity of said oscillator is repetitively updated.

5. In a system for generating frequency modulated signals, the combination of:

a variable frequency oscillator having a frequency control input and an output at which frequency modulated signals appear non-linearly in accord with a modulation signal at said input;

means for providing a modulating signal to said input whereby the system generates a frequency modulated signal at said output which, in the absence of compensation, is non-linearly related to such modulating signal;

selector means for causing said oscillator output to generate said frequency modulated signal about a selected one of a plurality of different carrier frequencies;

means for periodically measuring said non-linearities at each of said carrier frequencies to produce corresponding signals whose values indicate the corresponding nonlinearities and means for storing said values, whereby said values which are stored are repetitively updated; and

means responsive to said selector means for applying a compensating signal to said input in accord with that one of said stored signals which corresponds to the selected carrier frequency whereby the frequency modulated signal at said output is compensated for said non-linearity.

6. In a system for generating frequency modulated signals, the combination of:

a variable frequency oscillator having a frequency control input and an output at which frequency modulated signals appear non-linearly in accord with a modulation signal at said input;

means for providing a modulating signal to said input whereby the system generates a frequency modulated signal at said output which, in the absence of compensation, is non-linearly related to such modulating signal;

selector means for causing said oscillator output to generate said frequency modulated signal within a selected one of a plurality of different carrier frequency ranges;

means for periodically measuring said nonlinearities at each of a selected plurality of carrier frequencies lying respectively within one of said carrier frequency ranges to produce corresponding signals whose values indicate the corresponding nonlinearities and means for storing said values, whereby said values which are stored are repetitively updated; and

means responsive to said selector means for applying a compensating signal to said input in accord with that one of said stored signals which corresponds to the selected carrier frequency range whereby the frequency modulated signal at said output is compensated for said non-linearity.